Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sun Jul 02 2023 Valid 12Z Wed Jul 05 2023 - 12Z Sun Jul 09 2023 ...Major southern tier heat wave to ease this week, but may start to rebuild over the southern High Plains and vicinity next weekend... ...Overview... Not a lot has changed over the past day regarding the forecast pattern evolution from midweek through next weekend. A weak shortwave will depart from the Northeast on Wednesday and take its associated frontal system with it, aside from the trailing part of the front that will linger over the southern Mid-Atlantic. Farther west an upper trough extending from Canada into the northern Plains as of Wednesday will progress eastward with time, pushing a central U.S. front toward the East Coast and southern states. Progression of this front will produce a broad area of showers/thunderstorms over the central-eastern states over the course of the period. In addition, persistence of the western part of the front over the Plains and its eventual return northeast as a warm front may help to generate more locally heavy convection over the central Plains late this week into the weekend. Meanwhile consensus maintains the idea of a West Coast upper trough (with possible embedded low) settling over the West Coast from Thursday onward in response to an eastern Pacific ridge. An upper high over the southern Rockies/High Plains through the period could begin to strengthen slightly next weekend, with a somewhat better defined Interior West/Rockies ridge extending to the north-northwest late week into the weekend as the West Coast trough becomes established. ...Guidance Evaluation/Predictability Assessment... Once again an operational model composite early in the period followed by some inclusion of latest ensemble means (18Z GEFS/12Z ECens in the case of this update) with the models by the latter half of the forecast reflected the agreeable themes of the forecast and yielded only minor adjustments from continuity. One embedded detail uncertainty to monitor is with northern tier shortwave energy that may support an Upper Midwest/Upper Great Lakes frontal wave around Wednesday. ECMWF runs have been on the stronger/westward side for multiple runs (with corresponding heavier/westward QPF) while the CMC is on the weak side. The new 00Z GFS has trended somewhat stronger than earlier runs but the new ECMWF has trended weaker, favoring an intermediate solution. Going farther out in time, the blend/mean approach helps to smooth out the low-predictability shortwave differences that affect timing of the front as it heads into the eastern/southern U.S. Note that the 00Z UKMET seems to stray to the fast side for weak shortwave energy that other guidance shows over the Southeast by Friday. There are still low-confidence detail uncertainties within the upper trough developing near the West Coast but at least the latest GFS runs are generally closer to the model/mean consensus in principle than they were 24 hours or so ago. There are also meaningful differences for shortwave energy rotating around the larger scale Canadian/northern tier U.S. upper trough by next weekend, lowering confidence in specifics of a front that could eventually reach south of the Canadian border. ...Weather/Hazards Highlights... The wavy cold front progressing from the Upper Midwest/central Plains into the eastern/southern U.S. will likely produce multiple episodes of showers and thunderstorms, with some locally heavy. The Days 4-5 Excessive Rainfall Outlooks covering the Wednesday through Thursday night time frame will depict a broad Marginal Risk area from the western Great Lakes through the Mid/Upper Mississippi Valley and central/south-central Plains (Day 4) and Ohio Valley south-southwest to the Gulf Coast (Day 5). At this time guidance signals are not yet coherent enough for any Slight Risk areas but one or more could eventually arise if short range models begin to cluster better. Frontal wave specifics will play a role in how much rain falls over the Upper Midwest and vicinity around midweek. By Thursday-Thursday night, guidance is signaling potential for locally heavy convection to develop over the central Plains as the trailing part of the front stalls to the south and west of the region. The Day 5 outlook will reflect a Marginal Risk for now, with a Slight Risk possible in the future if guidance begins to converge for the axis of heaviest rainfall. Meanwhile the Days 4-5 outlooks introduce a Marginal Risk area mainly over the Carolinas, where especially on Day 4 latest solutions show a decent combination of instability, anomalous moisture, and corresponding locally heavy QPF, along and ahead of a stalling surface front. The front may weaken some by Day 5 but the moisture and instability should persist. Beyond Thursday night, expect locally heavy rainfall to continue eastward toward/through the East Coast into the weekend while the central Plains should see additional convective systems with locally heavy rainfall. Excessive heat across the southern tier should become less widespread for a time this week as upper ridging returns closer to climatological strength and latitude. The best potential for maximum heat index values to reach 105-110F or so mid-late week should be more confined to the far Southeast/Florida and central Gulf Coast through southern Texas. However upper ridging over the southern Rockies/High Plains may begin to strengthen next weekend and that would lead to temperatures gradually rising again over and near the southern Plains. Over the West, the greatest anomalies will be over the Pacific Northwest with highs up to 15-20F above normal into midweek (with some daily record highs possible) followed by gradual moderation as upper troughing sets up along the West Coast. Meanwhile, to the north of the Plains cold front expect unseasonably cool highs of 10-20F below normal over the northern-central High Plains especially on Wednesday, with gradual moderation leading to single-digit negative anomalies by the weekend. The leading warm sector will bring above normal temperatures from the Midwest into Northeast mid-late week, followed by near or slightly below normal readings after frontal passage. Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw